Sound reproducing system



P 5, 1939- J. R. BALSLEY 2,171,969

SOUND REPRODUCING SYSTEM Filed Feb. 4, 19157 Amplifier Patented Sept. 5, 1939 UNITED STATES SOUND REPRODUGING SYSTEM James R. Balsley, New York, N. Y., assignor, by mesne assignments, to Ozaphane Corporation of America, Stamford, Conn., a corporation of Delaware Application February 4, 1937, Serial No. 123,972

15 Claims.

This invention relates to the reproduction of sound from a sound record and more particularly to the reproduction of sound from a film having push-pull photographic sound tracks thereon.

An object of the invention is to reduce the relative noise level in the reproduced sound.

Another object is to provide a novel and improved system for increasing the volume range of the reproduced signal with respect to that of the record.

Another object is to more fully utilize the characteristics of a push-pull sound record of the so-called noiseless type in which the average transmission is proportional to the signal strength.

Another object is to provide a novel and improved system of the type above indicated.

Other objects and advantages will be apparent as the nature of the invention is more fully disclosed.

This invention is particularly applicable to a sound reproducing system including a push-pull photographic sound record having a pair of sound tracks thereon, which are both complete records but are out of phase, and are so recorded that the average transmission is proportional to the signal volume. This type of recording, by reducing the light transmission when the signal volume is low or during the silent periods, materially reduces the background noise which would otherwise be produced. The record is reproduced by a pair of photo-electric cells connected to a push-pull amplifier so that the Variations due to the changes in the average transmission characteristics of the record do not appear as extraneous noises in the output circuit.

A feature of the invention consists in utilizing the change in average transmission of the record to expand the volume range of the reproduced signal so as to compensate for the volume compression which is inherent in the record and to cause the reproduced signal to more nearly correspond to the original signal from which the record was made.

In one embodiment, the current to the photoelectric cells is passed through a resistance in the common grid return of the push-pull amplifier to produce a voltage drop which opposes the normal negative grid bias of the tubes so as to introduce a change in the amplification ratio of the amplifier proportional to the change in signal volume. The amount of expansion may be varied by changing the value of the control resistance and its efiect on the operation of the tubes. The control resistance is not afiected by the signal currents which cancel out in the common grid return. Hence, it produces volume expansion without introducing extraneous noises or other eifects into the system.

In a further embodiment the control effect is increased by using a multi-grid tube, such as a penta-grid tube, and applying the signal voltage and control voltage to one or more control'grids. In this way the output volume range can be made many times the recorded range, while the noise level is actually reduced.

Other features consist in the details of construction and combinations of parts hereinafter more fully described.

Although the novel features which are believed to be characteristic of this invention are pointed out more particularly in the claims appended hereto, the invention itself will be better understood by referring to the following description taken in connection with the accompanying drawing in which certain embodiments thereof have been set forth for purposes of illustration.

In the drawing:

Figure 1 is a schematic diagram illustrating a sound reproducing system embodying the present invention; and

Figure 2 is a similar schematic diagram illustrating the invention applied to a push-pull amplifier having multi-grid tubes.

Referring to the drawing more in detail, the invention is shown in Figure 1 as applied to a sound reproducing system comprising a pair of photoelectric cells it which are adapted to be actuated in accordance with the record tracks of a push-pull sound record, the average transmission of which is proportional to the signal volume. Records of this type, which are commonly called noiseless push-pull records, are well known in the art. Consequently, they are not shown in the present drawing. They may be of either the variable area or the variable density type. It is to be understood that the light passing through the individual record tracks causes suitable variation in the current passing through the photoelectric cells Ill.

In the form shown in Figure l, the positive terminals of the photo-electric cells I!) are connected to the positive side of a suitable source of potential, such as a battery II, the negative side of which is grounded.

The negative terminals of the photo-electric cells iii are connected to the input circuit of a push-pull amplifier comprisinga pair of space discharge devices Hi having cathodes 15, control grids i6 and anodes ii. The photo-electric cells are connected to the respective control grids it of the space discharge devices i i. Bridged across the input circuit of the push-pull amplifier are resistances 28 in series, the mid-point of which is connected through a resistance 2| to the ground, as at 22. The cathodes 15 are positively biased with respect to ground by a suitable source of biasing potential, not shown.

The output circuit of the amplifier comprises iii) III

a push-pull transformer 25, the primary 26 of which is connected to the anodes l1 and to a source of'space current, not shown. The secondary 28 of the transformer 25 may be connected to an amplifier 29 of any suitable type,

which is adapted to amplify to the desired value the signals received from the push-pull amplifier. The output circuit of the amplifier 29 is connected through an output transformer 30 to actuate a loud speaker 3| of any desirable type.

It is to be understood that the details of the various parts of the above-described system may be caried in accordance with accepted practice and only so much thereof has been shown as is necessary to an understanding of the present invention. The space discharge devices may be provided with the usual sources of operating potential, such as self-biasing resistors, bleeder resistors, batteries, or the like, and may include various screen grids and suppressor grids in addition to the control grids it. A triode has been shown merely for the purpose of illustration. Furthermore, the details of the amplifier and the loud speaker may be varied in accordance with the particular system.

In the operation of this system, current from the battery passes through the photo-electric cells ii! and through the resistances 20 and 2| in series, in the direction of the arrows. The signal variations of the record cause changes in current in the photo-electric cells l and in the resistances 29, thereby producing a signal voltage which is impressed on the grids N of the pushpull amplifier.

The direct current component,corresponding to the average transmission of the record, produces a voltage drop across resistances 20 and 2| which opposes the normal negative grid bias of the devices l4. Consequently, an increase in the average transmission of the record causes the amplification ratio of the space discharge devices 4 to increase and vice versa, the loud notes being amplified more in proportion than the weak notes. The range between loud and weak signals in the output of the push-pull amplifier is accordingly greater than the range of the record. This expansion in volume range is made such that the inherent volume compression of the sound record is compensated for and the reproduced signal is caused to correspond more nearly to that of the original sound from which the record was made.

The proper relationship between the signal and the expansion can be obtained by varying the relative values of the resistances 20 and 2| to produce the desired efiect. The resistance 2|, being in the common grid return, controls only the change in bias, whereas the resistances 28 control both the signal voltage and the bias. In some instances, reactances can be used instead of the resistances 26 so that the bias is dependent only upon the voltage drop in the resistance 2|.

The output from the push-pull amplifier is further amplified in the amplifier 29 and is impressed upon the loud speaker 3| in the usual manner.

In the above-described. systems, the noise level is reduced by the noiseless recording which cuts down the average transmission of the record on lower signal volumes. The use of a push-pull system eliminates extraneous noises which might otherwise be produced due to the operation of the noise reduction circuits.

The invention thus utilizes the variable transmission characteristics of a record made by the sound is obtained.

In the embodiment shown in Figure 2, the signal and control are applied to two grids of a penta-grid tube. In the form shown, a pair of photo-electric cells ill have their positive terminals connected to a suitable battery H, as described in connection with Figure 1. The negative terminals of these photo-electric cells are connected to the third grids 40 of a pair of penta-grid tubes 4| which are connected in pushpull relationship. Resistances 42 are bridged across the input circuit of the push-pull amplifier. A resistance 43 is connected between the mid-point of the resistances 42 and ground. The mid-point of the resistances 42 is also connected to the first grids 45 of the penta-grid tubes 4|.

The cathodes 5B of the tubes 4| are positively biased with respect to ground by a suitable source of biasing potential, not shown. The second and fourth grids 52 and 53 respectively which constitute the screen grids are connected together and to a source of positive screen potential. The fifth grids 55, which constitute the suppressor grids, are connected to the cathodes 50. The anodes 51 are connected to the primary 58 of a push-pull transformer 59, the mid-point of which is connected to a source of space current. The secondary 68 of the transformer 59 is connected to the input circuit of an amplifier 6| which may beof any standard type and may include one or more stages of amplification. The output circuit of the amplifier SE is connected through an output transformer 52 to a loud speaker 53 which may be of any standard type.

The details of this system may be varied in accordance with accepted practice and the usual sources of operating potential may be provided, as set forth in connection with Figure 1. Only so much of the system has been shown as is necessary to an understanding thereof.

The operation of this system is generally similar to that of Figure l. The signal voltage is applied to third grids 4B and the control voltage is applied to both the first grids 45 and the third grids 48. The normal negative bias on the grids 45 is opposed by the voltage drop in the resistance 43 and the normal negative bias on the grids 4B is opposed by the voltage drop in the resistances 42 and 43. The grids 40 operate less negative than the grids 45 because of the voltage drop in the resistances 42 and are varied over wider limits.

It is to be understood that suitable changes can be made in the values of the resistances 42 and 43 to obtain the desired efiect or variable resistances can be employed which may be connected to vary the control effect without changing the normal bias on the grids due to constant current through the resistances. Reactances can be used in place of the resistances 42, if desired, in which case, the control voltage will be applied only to the first grids 45 and only signal voltage will be applied to the third grids 40, so that one set of grids controls the signal frequency component in the output circuit and the other set of grids is varied in accordance with the envelope of the signal current to control the signal volume. In some types of tubes it may be desirable to reverse the connections to the first and third grids to apply the control voltage to the first grids and the signal voltage to the third grids. Incertain instances the control voltage may be applied to electrodes other than the grids. The change in grid bias is preferably selected to increase the amplification of the tubes an amount corresponding to the volume compression in the sound record, whereby the output signal is caused to more nearly represent the original signal from which the record was made.

By applying the control to two or more control grids, the amount of expansion may be varied within wide limits. If the mutual conductance of the tubes is different for the different grids, the output volume range may be made many times the recorded range with a proportional reduction in noise.

The control voltage which is utilized for varying the grid bias may be obtained in various ways. It has been shown herein as obtained by the use of a resistance in the common grid return of a push-pull amplifier because of the simplicity of this arrangement. Furthermore, the control voltage may be used to vary the normal bias on a plurality of grids and may be applied to other types of space discharge devices. The signal voltage may also be applied to two or more grids which may be the same grids or different grids from the expansion control grids. Certain wellknown types have been described merely for purpose of illustration.

The invention, with suitable modifications, may be applied to other types of systems having variable transmission characteristics, such as a highfrequency transmission system having a variable carrier.

While certain specific embodiments have been shown for purposes of illustration, it is to be understood that various changes and modifications may be made therein as will be apparent to a person skilled in theart. The invention is accordingly to be limited only by the following claims when interpreted in view of the prior art.

I claim:

1. In a photo-electric control system for reproducing signals derived from a pair of light beams modulated respectively by signals having push-pull relationship, said light beams varying in average intensity in accordance with the signal strength, a push-pull amplifier, photo-electric means responsive to said light beams and connected to apply a signal voltage to said amplifier for amplification, and means responsive to the average intensity of said light beams to vary the amplification ratio of said amplifier.

2. In a photo-electric control system for reproducing signals derived from a pair of light beams modulated respectively by signals having push-pull relationship, said light beams varying in average intensity in accordance with the signal strength, a push-pull amplifier, photo-electric means responsive to said light beams and connected to apply a signal voltage to said amplifier for amplification, a resistance connected to control the amplification ratio of said amplifier, and means producing a voltage drop in said resistance proportional to the average intensity of said light beams whereby the amplification ratio of said amplifier is varied in accordance with changes in signal volume.

3. In a photo-electric control system for reproducing signals derived from a pair of light beams modulated respectively by signals having push-pull relationship, said light beams varying in average intensity in accordance with the signal strength, a push-pull amplifier, photo-electric means responsive to said light beams and connected to apply a signal voltage to said amplifier-for amplification, a resistance connected to control the amplification ratio of said amplifier, and means passing through said resistance, the current drawn by said photo-electric means to develop a voltage drop therein which varies in accordance with the average intensity of said lightbeams whereby the amplification ratio of said amplifier is varied in accordance with changes in signal volume.

4. In a photo-electric control system for reproducing signals derived from a pair of light beams modulated respectively by signals having push-pull relationship, said light beams varying in average intensity in accordance with the signal strength, an amplifier comprising a pair of space discharge devices connected in push-pull relationship, a pair of photo-electric cells responsive to said light beams and connected respectivelyto the input circuit of said space discharge devices to supply signal voltage thereto, a resistance connected to control the grid biasing potential of said devices, and means passing through said'resistance, the current drawn by both of said. photo-electric cells to develop a voltage drop therein which varies in accordance with the average intensity of said light beams, whereby the amplification ratio of said amplifier is varied in accordance with changes in signal volume.

5. In a system for reproducing sound from a pair of push-pull sound record tracks having variable average transmission characteristics, a pair of photo-electric cells separately energized in accordance with said push-pull record tracks, an amplifier comprising a pair of space discharge devices connected in push-pull relationship to be energized by said photo-electric cells, a translating device actuated by said amplifier, and means to vary the amplification characteristics of said amplifier in accordance with changes in the average transmission characteristics of said record for changing the volume range of the reproduced signal.

6. In a system for reproducing sound from a pair of push-pull sound record tracks having variable average transmission characteristics, a pair of photo-electric cells separately energized in accordance with said push-pull record tracks, an amplifier comprising a pair of space discharge devices connected in push-pull relationship to be energized bysaid photo-electric cells, a translating device actuated. by said amplifier, and means responsive to an increase in the average transmission characteristics of said record for increasing-the amplification ratio of said amplifier and vice versa so that the volume range of the reproduced signal is expanded over the recorded volume range.

'1. In a system for reproducing sound from a pair of push-pull sound record tracks having variable average transmission characteristics, a pair of photo-electric cells separately energized in accordance With said push-pull record tracks, an amplifier comprising a pair of space discharge devices connected in push-pull relationship to be energized by saidphoto-electric cells, a trans lating device actuated by said amplifier, and resistance means connected in series with said photo-electric cells to develop a voltage drop proportional to the average current passed through said cells, said resistance means being connected to oppose the normal negative grid bias of said devices so as to vary the amplification ratio thereof in accordance with the average signal strength.

8. In a system for reproducing sound from a pair of push-pull sound record tracks having variable average transmission characteristics, a pair of photo-electric cells separately energized in accordance with said push-pull record tracks, an amplifiercomprising a pair of space discharge devices connected in push-pull relationship to be energized by said photo-electric cells, a translating device actuated by said amplifier, and a resistance connected in series with said photoelectric cells to develop a voltage drop proportional to the average current passed through both of said cells, said resistance being connected in the common grid return of said space discharge devices to oppose the normal negative grid bias thereof so as to vary the amplification ratio of said amplifier in accordance with the average signal strength.

9. In a system for reproducing sound from a pair of push-pull sound record tracks having variable average transmission characteristics, a pair of photo-electric cells separately energized in accordance with said push-pull record tracks, an amplifier comprising a pair of space discharge devices connected in push-pull relationship to be energized by said photo-electric cells, a translating device actuated by said amplifier, said space discharge devices each having a control grid, a pair of impedances connected across the input circuit of said amplifier to control the signal voltage impressed upon said control grids, and a resistance connected to the common point of said impedances and in the common grid return of said amplifier, said resistance carrying the current which passes through both of said photoelectric cells to develop a voltage drop proportional to the average transmission of said record tracks and being connected to oppose the normal negative grid bias of said space discharge devices whereby the amplification ratio of said amplifier is varied in accordance with the average signal strength.

10. In a system for reproducing sound from a pair of push-pull sound record tracks having Variable average transmission characteristics, a pair of photo-electric cells separately energized in accordance with said push-pull record tracks, an amplifier comprising a pair of space discharge devices connected in push-pull relationship to be energized by said photo-electric cells, a translating device actuated by said amplifier, said space discharge devices each having a plurality of control grids, means impressing signal voltage derived from said cells upon one of said control grids, and means varying the bias potential on both of said control grids in accordance with the average signal strength, whereby the amplification characteristics cf said devices are changed accordingly.

11. In a system for reproducing sound from a pair of push-pull sound record tracks having variable average transmission characteristics, a pair of photo-electric cells separately energized in accordance with said push-pull record tracks, an amplifier comprising a pair of space discharge devices connected in push-pull relationship to be energized by said photo-electric cells, a translating device actuated by said amplifier, said space discharge devices each having a plurality of control grids, means impressing a signal voltage derived from said cells upon at least one of said control grids, means applying normal negative bias potentials to both of said control grids, and means responsive to the average signal intensity to oppose said negative bias, whereby the amplification ratio of said space discharge devices is varied in accordance with the signal strength.

12. In a system for reproducing sound from a pair of push-pull sound record tracks having variable average transmission characteristics, a pair of photo-electric cells separately energized in accordance with said push-pull record tracks, an amplifier comprising a pair of space discharge devices connected in push-pull relationship to be energized by said photo-electric cells, a translating device actuated by said amplifier, said space discharge devices each having a plurality of control grids, a pair of impedances connected across the input circuit of said amplifier to control the signal voltage impressed upon one of said control grids, and a resistance connected to the common point of said impedance and in the common grid return of said amplifier, said resistance carrying the current which passes through both of said photo-electric cells to develop a voltage drop proportional to the average transmission of said record tracks, said resistance being connected to control the bias on the other of said control grids, whereby the amplification ratio of said space discharge devices is varied in accordance with the signal strength,

13. The method of reproducing sound from a photographic sound record comprising a air of push-pull sound'record tracks having variable average transmission characteristics, which comprises scanning said record tracks to derive a pair of signal voltages therefrom having push-pull relationship, amplifying said signal voltages in a push-pull amplifier, deriving a control voltage from the combined effect of the variations in average transmission characteristics of both of said record tracks, and utilizing said control voltage to vary the amplification ratio of said pushpull amplifier so as to expand the volume range of the reproduced signal in accordance with the variations in average transmission characteristics of said record.

14. In a system for reproducing signals derived from a pair of sound track carriers respectively carrying signals having push-pull relationship, said carriers varying in average intensity in accordance with the signal strength, a push-pull amplifier, means to derive signal voltages from said carriers and to apply said voltages to said amplifier for amplification, and means responsive to the average intensity of said carriers to vary the amplification ratio of said amplifier so as to vary the amplitude range of the reproduced signal.

15. In a system for reproducing signals derived from a pair of photographic sound records modulated respectively with signals having pushpull relationship, said photographic sound records Varying in average intensity in accordance with the signal strength, a push-pull amplifier, means to derive signal voltages from said photographic sound records and to apply said voltages to said amplifier for amplification, and means responsive to the average intensity of said photographic sound records to vary the amplification ratio of said amplifier so as to vary the amplitude range of the reproduced signal.

JAMES R. BALSLEY.

CERTIFICATE OF CORRECTION. Patent No. 2,171,969. September 5, 1959.

' JAMES R. BALSLEY. I It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 1, second column, line no, for "variation" read variations; page 2, first column, line 10, for "desirable" read suitable; line 13, for "caried" read varied; page 1;, second column, line 52, claim 15, for "air" read pair; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 3rd day of October, A. D. 1959.

Henry Van A redale,

(Seal) Acting Commissioner of Patents. 

